This invention relates to an optical multiplexer-demultiplexer.
The development of optical fibre telecommunications involves the development of numerous components and the improvement of their performances.
This is the case of wavelength multiplexer-demultiplexers that are used for transmitting a very large number of channels via the same fibre.
In such a multiplexer, the incoming luminous fluxes, with different wavelengths, regularly spaced apart, are superimposed in order to generate a multiwavelength flux that will be transmitted by the fibre.
Conversely, at the output of the transmission fibre, a demultiplexer receives the multiwavelength composite flux and generates separate single wavelength fluxes.
The characteristic performances of these multiplexers and/or demultiplexers are the number of channels capable of being transmitted via the same fibre without any risks of crosstalk, i.e. spurious effects generated by one of the channels onto its neighbours and with as low an attenuation as possible, but especially whose amplitude is independent from the wavelength.
To that end, multiplexers and demultiplexers implementing a diffraction grating can be realised.
In particular, for a plane grid with pitch p used in a Littman-Metcalf configuration, a collimated luminous flux, incident by a wavelength xcex tilted with respect to the normal of the grid of an angle xcex81 is scattered on a dispersion plane perpendicular to the lines of the grid in the form of a collimated beam with a direction tilted by an angle xcex82 with respect to the normal of the grid, xcex81 and xcex82 being connected by the relation p sin xcex81+p sin xcex82=xcex; the mirror of the Littman-Metcalf configuration is then perpendicular to the direction xcex82. The transfer function of the resulting multiplexer-demultiplexer is periodic and composed of peaks whose width at the apex is relatively narrow. This implies that the wavelengths, which do not coincide exactly with the apex of these peaks, may undergo variable attenuations, which is particularly disturbing with systems where global amplification of the multiplexed flux takes place. Therefore, it has been sought to widen the shape at the apex of the peaks of the transfer function even if it is detrimental to the attenuation of the transmitted flux.
We also know the operation of the two-wave interferometers, such as the Fizeau interferometer also known as the little accurate Fabry-Perot interferometer, to which the following description will be referred for non limited exemplification purposes. They comprise two plane mirrors, partially transparent, which form together a cavity. It is known that the spectral response of such Fizeau interferometer is also periodic, that its periodicity depends on the spacing e between the mirrors.
It is known that the use of a Fizeau interferometer with transfer function of the same pitch as that of the multiplexer enables to widen the shape of the peak at its apex.
More precisely, it is known to implement, with the diffraction grating, a Fizeau interferometer with a spectral response of the same pitch as that of the response of the multiplexer-demultiplexer. The Fizeau interferometer diminishes the maximum transmission coefficient, but widens the transmission peaks of the transfer function, at their maximum.
The frequency spacing between two consecutive peaks of the transfer function, of the interferometer is xcex94fFSR (FRS for Free Spectral Range) and this spacing is linked to the thickness e of the Fizeau interferometer by the formula
xcex94fFSR=cxc3x971/(2e)
where c corresponds to the speed of light.
From the value of xcex94fFSR, the thickness of the Fizeau interferometer can therefore be deduced.
e=c/(2 xcex94fFSR)
We also know that a Fizeau interferometer whose mirrors have a reflection coefficient R that is relatively small, the modulation depth, in intensity, in relation to the frequency, of the flux transmitted is approximately 1-4R.
To sum up, as xcex94fWDM is the pitch of the response by the multiplexer, or the interchannel space fixed by the telecommunications system, currently 100 GHz:
xcex94fWDM=xcex94fFSR implies e=1.5 mm
It can then be noticed that the optimal widening of the transfer function of the diffraction grating/interferometer assembly is obtained by a modulation depth (I0-I1)/I0 in the order of 60%.
The purpose of this invention is to realise a multiplexer or a demultiplexer with flattened response, i.e. that is able to transmit signals over a large number of channels, while reducing crosstalk and ensuring, by signal loss, uniform attenuation for all the transmitted channels with enhanced performances with respect to the use of the Fizeau interferometer described above.
The aim of the corresponding improvement was to obtain a uniform transmission spectral range, for every peak, of the width comparable to that of the systems of the previous art, with reduced attenuation, i.e. with a better transmission coefficient.
To that effect, the invention relates to an optic multiplexer or demultiplexer comprising an inlet optic fibre, a wavelength dispersive optic system, outlet optic fibres and a two-wave interferometer.
According to the invention, the two-wave interferometer has a spectral response exhibiting minima at twice the response frequency of the multiplexer or demultiplexer.
As a result of the claimed configuration, the attenuation generated by the Fizeau filter shows a curvature close to the reverse of that of the transmission function of the grating. The curvatures generate by reciprocal compensation a resulting composite function whose transmission coefficient, close to its maximum, is relatively uniform that makes it look rather like a strobe function. This result is obtained while preserving good transmission of the whole system. The transmission function of the two-wave interferometer (preferably a Fizeau filter) also exhibits a minimum between two maxima of the transfer function of the multiplexer. The minimum corresponds then to a minimum of that transfer function of the multiplexer and therefore does not affect the latter. The efficiency of the filter is then boosted without any significant shortcoming.
This invention also relates to the characteristics that will become evident with the following description and that will be considered individually or in all their technically possible combinations:
the interferometer is a Fizeau interferometer,
the inlet and output fibres are single-mode fibres,
the reflector of the dispersive system is plane,
the multiplexer operates in the 1525-1625 nm range,
the multiplexer processes approx. 16 to 128 channels spaced by 50 or 100 GHz.